CRYSTAL CHEMISTRY OF MINERALS OF THE WÖHLERITE GROUP FROM THE LOS ARCHIPELAGO, GUINEA

593

The Canadian Mineralogist Vol. 50, pp. 593-609 (2012) DOI : 10.3749/canmin.50.3.593

CRYSTAL CHEMISTRY OF MINERALS OF THE WÖHLERITE GROUP FROM THE LOS ARCHIPELAGO, GUINEA

Cristian BiaGiOni§ and stefanO MerLinO

Dipartimento di Scienze della Terra – Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy

Gian CarLO ParOdi

Laboratoire de Minéralogie et Cosmochimie du Muséum UMR 7160, Muséum National d’Histoire Naturelle, 61 rue Buffon, F-75005 Paris, France

nataLe PerCHiaZZi1

Dipartimento di Scienze della Terra – Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy

aBstraCt

As part of the SYNTHESYS European project, a suite of samples from the nepheline syenites of the Los Archipelago, Guinea, was studied in order to characterize the crystal chemistry of the minerals belonging to the wöhlerite group. Chemical and X-ray studies permitted the identification of four different mineral species: låvenite, normandite, wöhlerite, and hiortdahlite I. The identification of hiortdahlite II reported by Robles et al. (2001) seems to be questionable.

Låvenite and normandite occur in the agpaitic suite, in association with analcime, catapleiite, fluorite, mosandrite, sérandite, sphalerite, and villiaumite; wöhlerite and hiortdahlite I are associated with britholite-(Ce), magnetite, titanite, and zircon.

All the studied låvenite and normandite crystals are intermediate terms in the låvenite-normandite series. Wöhlerite shows the highest Mn content ever recorded for this species. Hiortdahlite I is the poorest in terms of both Zr and Na content and the richest in Ca and F among the known occurrences of “hiortdahlite”.

The crystal structures of these four phases were refined, with R factors ranging from 1.9% (for Mn-rich wöhlerite) to 7.3% (for hiortdahlite I).

The wide range in Zr-Ti ratios observed in the låvenite-normandite series using new data and all available chemical data, not reported in other Zr-Ti minerals, seems to be related to the contemporaneous substitution of Mn by Ca.

The comparison of the geometry of (Nb,Ti)- and Zr-centered polyhedra, including all the available structural studies of the wöhlerite group, indicates it is possible to make a distinction between (Nb,Ti)-dominant and Zr-dominant sites in this mineral family.

Keywords: låvenite, normandite, wöhlerite, hiortdahlite I, agpaitic syenite, Los Archipelago, Guinea

§ E-mail address: [email protected]

intrOduCtiOn

The Los Archipelago, located 5 km offshore of Conakry, capital of Guinea, is mainly formed by neph-eline syenites emplaced in the West African continental margin during Albian time (Moreau et al. 1996). This magmatic complex is similar to other alkaline massifs, such as Ilímaussaq in south Greenland; Mont Saint-Hilaire, Québec, Canada; and the Khibiny and Lovozero massifs in Kola Peninsula, Russia.

The mineralogy of the Los Archipelago was studied by Lacroix (Lacroix 1911), who first described villiau-

mite (Lacroix 1908) and sérandite (Lacroix 1931). Parodi & Chevrier (2004) reported the occurrence of some unknown Zr-Ti silicates; one of them was subsequently described as the new mineral roumaite by Biagioni et al. (2010a). The Zr-Ti-Nb-REE disilicates from the Los Archipelago belong to two distinct groups of minerals, i.e., the wöhlerite group and the mosandrite group. As shown in Table 1, both groups are charac-terized by the general formula M8(Si2O7)2(O,OH,F)4, where M represents a wide range of cations with variable charges and ionic radii, typically six- to eight-fold coordinated. As pointed out by Bellezza et al.

594 tHe Canadian MineraLOGist

(2004b), the above general formula can also describe the chemistry of another group of disilicates, i.e., the rosenbuschite group.

These three groups of minerals differ for their structural arrangements (Fig. 1), that can be described in modular terms. The crystal structure of the members of the wöhlerite group (Fig. 1a) is formed by two kinds of modules, i.e., walls of “octahedra”, four columns large and extending along [001], and disilicate groups (Merlino & Perchiazzi 1988). In the rosenbuschite group (Fig. 1b), besides disilicate groups, “octahedral” cations are arranged by edge sharing into layers (O layers) and ribbons. Heterogeneous layers (H layers) formed by octahedral ribbons and disilicate groups alternate with the O layers, into a layered HOH structure (Christiansen et al. 2003b). Lastly, minerals belonging to the mosandrite group (Fig. 1c) are characterized by three kinds of modules: (1) layers of seven-fold coor-dinated polyhedra, analogous to those occurring in the minerals of the tobermorite group (Merlino et al. 1999); (2) “octahedral” layers, resembling the O layers of rosenbuschite group; and (3) disilicate groups.

The minerals of the three above mentioned groups are typically found as rare accessories in agpaitic syenites and carbonatites. They are usually identi-fied only on the basis of EDS and WDS chemical analyses; as an example, see the studies by Ferguson (1978), Woolley & Platt (1986), Mariano & Roeder (1989), Keller et al. (1995), and Coulson (1997). In view of the wide chemical variability inside each of these mineral groups, an unambiguous identification is instead possible only by coupling chemical data with X-ray diffraction studies. X-ray powder diffraction data can readily indicate the mineral group to which a given phase belongs, whereas, owing to the modular character of these phases, a full identification of the mineral species is possible only through single-crystal X-ray diffraction studies coupled with chemical analyses.

As part of the SYNTHESYS European project (FR-TAF-2137: “Crystallography and crystal chemistry of rare Zr-Ti-Nb phases from the Iles de Los, Guinea”), we investigated a suite of samples kept in the Muséum National d’Histoire Naturelle of Paris and labeled as members of the wöhlerite group from the Los Archi-

TABLE 1. MINERAL SPECIES BELONGING TO THE MOSANDRITE, ROSENBUSCHITE, AND WÖHLERITE GROUPS

Name Formula a (Å) b (Å) c (Å) α (°) β (°) γ (°) s.g. Z Ref.

mosandrite groupdovyrenite Ca6Zr[Si2O7]2(OH)4 5.666 18.844 3.728 Pnnm 1 [1]mosandrite Ti(,Ca,Na)3(Ca,REE)4(Si2O7)2[H2O,OH,F]4∙~

1H2O7.398 5.595 18.662 90 101.37 90 P21/c 2 [2]

nacareniobsite-(Ce)

NbNa3Ca3REE(Si2O7)2OF3 7.468 5.689 18.891 90 101.37 90 P21/c 2 [3]

rinkite TiNa2Ca4REE(Si2O7)2OF3 7.433 5.660 18.818 90 101.35 90 P21/c 2 [4]roumaite (Nb,Ti)(Ca,Na,)3(Ca,REE)4(Si2O7)2(OH)F3 7.473 11.294 18.778 90 101.60 90 Cc 4 [5]

rosenbuschite groupgötzenite NaCa6Ti(Si2O7)2OF3 9.619 5.725 7.331 89.92 101.13 100.64 P-1 1 [6, 7]grenmarite Na4MnZr3(Si2O7)2O2F2 5.608 7.139 18.575 90 102.60 90 P2/c 2 [7, 8]hainite Na2Ca4(Y,REE)Ti(Si2O7)2OF3 9.584 7.267 5.708 89.85 101.22 101.03 P-1 1 [6, 7]kochite Na3Ca2MnZrTi(Si2O7)2OF3 10.032 11.333 7.202 90.19 100.33 111.55 P-1 2 [7, 9]rosenbuschite Na6Ca6Zr3Ti(Si2O7)4O2F6 10.137 11.398 7.271 90.22 100.31 111.87 P-1 1 [6, 7]seidozerite Na4MnZr2Ti(Si2O7)2O2F2 5.556 7.075 18.406 90 102.71 90 P2/c 2 [6, 7]

wöhlerite groupbaghdadite Ca6Zr2(Si2O7)2O4 10.432 10.163 7.356 90 90.96 90 P21/a 2 [10]burpalite Na4Ca2Zr2(Si2O7)2F4 10.117 10.445 7.256 90 90.04 90 P21/a 2 [11]cuspidine Ca8(Si2O7)2F4 10.906 10.521 7.518 90 109.30 90 P21/a 2 [12]hiortdahlite I (Na,Ca)2Ca4Zr(Mn,Ti,Fe)(Si2O7)2(F,O)4 11.015 10.941 7.353 109.35 109.88 83.43 P-1 2 [13]hiortdahlite II Na2Ca4Zr(Y,Zr,Mn,Fe)(Si2O7)2(F,O)4 11.012 10.342 7.359 89.92 109.21 90.06 P-1 2 [14]janhaugite Na3Mn3Ti2(Si2O7)2(O,OH,F)4 10.668 9.787 13.931 90 107.82 90 P21/n 4 [15]låvenite (Na,Ca)4(Mn,Fe)2(Zr,Ti,Nb)2(Si2O7)2(O,F)4 10.83 9.98 7.174 90 108.1 90 P21/a 2 [16]marianoite Na2Ca4(Nb,Zr)2(Si2O7)2(O,F)4 10.846 10.226 7.273 90 109.33 90 P21 2 [17]niocalite Ca7Nb(Si2O7)2O3F 10.863 10.431 7.370 90 110.1 90 Pa 2 [18]normandite Na2Ca2(Mn,Fe)2(Ti,Nb,Zr)2(Si2O7)2O2F2 10.799 9.801 7.054 90 108.08 90 P21/a 2 [19]wöhlerite Na2Ca4Zr(Nb,Ti)(Si2O7)2(O,F)4 10.823 10.244 7.290 90 109.00 90 P21 2 [20]

[1] Kadiyski et al. 2008; [2] Bellezza et al. 2009; [3] Sokolova & Hawthorne 2008; [4] Cámara et al. 2011; [5] Biagioni et al. 2010a; [6] Christiansen et al. 2003b; [7] Sokolova 2006; [8] Bellezza et al. 2004a; [9] Christiansen et al. 2003a; [10] Biagioni et al. 2010b; [11] Merlino et al. 1990; [12] Saburi et al. 1977; [13] Merlino & Perchiazzi 1985; [14] Merlino & Perchiazzi 1987; [15] Annehed et al. 1985; [16] Mellini 1981; [17] Chakhmouradian et al. 2008; [18] Mellini 1982; [19] Perchiazzi et al. 2000; [20] Mellini & Merlino 1979.

CrystaL CHeMistry Of tHe wöHLerite GrOuP frOM tHe LOs arCHiPeLaGO, Guinea 595

pelago, characterizing them both through chemical and X-ray diffraction studies, so to ascertain the true nature of these minerals.

MineraLs Of tHe wöHLerite GrOuP: an Overview

The wöhlerite group (so-called from the first described mineral species, wöhlerite; Scheerer 1843) includes eleven minerals (Table 1) with monoclinic or triclinic symmetry. As stated above, the crystal structure of these phases can be described as the assemblage of two distinct modules.

The walls of “octahedra” are interconnected both directly by corner-sharing and through Si2O7 groups, each group being linked to three walls. The various minerals differ according to two basic features: (1) the different way of connecting the disilicate groups to the walls of “octahedra” and (2) the cation distributions within the walls (Merlino & Perchiazzi 1988).

Merlino & Perchiazzi (1988) showed that within unit-cell dimensions of ~10 × ~10 × ~7.3 Å3, ten different topological structure types exist, corre-sponding to the ten distinct ways in which the disilicate groups can be connected to the framework of octahedra; these ten structure types can be distributed among four distinct types of unit-cell. The actual space group symmetry depends on the topological symmetry and the cation distribution within the walls of “octahedra” (Merlino & Perchiazzi 1988).

Minerals belonging to the wöhlerite group are common accessory phases in nepheline syenites from localities worldwide, i.e., Poços de Caldas, Brazil (Atencio et al. 1999); Mont Saint-Hilaire (Horváth & Gault 1990) and Varennes (Horváth et al. 1998), both in Québec, Canada; the Eifel volcanic region, Germany (Hentschel 1986); Langesundsfjord, Norway (Brögger 1890); the Agua de Pau volcano, Azore Islands (Ridolfi et al. 2003); and the Khibiny and Lovozero massifs,

fiG. 1. Crystal structures of the phases belonging to the wöhlerite (a), rosenbuschite (b), and mosandrite (c) groups.


Russia (Vlasov et al. 1966). Regarding the occurrence of minerals of the wöhlerite group in the Los Archi-pelago, several investigators (Lacroix 1924, Lazarenkov 1975, Moreau et al. 1996, Robles et al. 2001) described their occurrence in the agpaitic facies of the syenites.

identifiCatiOn Of tHe sPeCiMens frOM LOs arCHiPeLaGO

Description of samples

The crystal morphology of the minerals belonging to the wöhlerite group from the Los Archipelago ranges from prismatic to acicular, always with a good prismatic cleavage. According to differences in their colors, three types of phases have been distinguished: (1) Type A: prismatic to acicular crystals, up to 1 cm in length, with color ranging from honey-yellow to orange. In some cases, crystals form radial aggregates. This type corresponds to specimens labeled as låvenite (speci-mens 106.211 and 131.80) and LOS 180; (2) Type B: yellow prismatic crystals, up to 0.8 mm long, labeled as wöhlerite (specimens 111.6 and 113.119); (3) Type C: whitish prismatic crystals, labeled as “hiortdahlite” (specimen 119.115). This specimen is the same as that studied by Robles et al. (2001) and identified as hiortdahlite II on the basis of X-ray powder diffraction data alone.

The crystals of type A are associated with analcime, catapleiite, fluorite, mosandrite, sérandite, sphalerite, and villiaumite, a typical mineral assemblage for the agpaitic suite (Moreau et al. 1996). On the contrary, grains of type B and C are associated with britholite-(Ce), magnetite, titanite, and zircon.

Preliminary X-ray diffraction studies and qualitative chemical analyses

The crystals were preliminarily tested by X-ray powder diffraction using a 114.6 mm Gandolfi camera and Ni-filtered CuKα radiation. Powder patterns clearly indicated that the investigated crystals belong to the wöhlerite group. Subsequent X-ray single-crystal studies were performed using the Weissenberg tech-nique, collecting, as suggested by Merlino & Perchiazzi (1988), photographs of the reciprocal layers hk0 and hk1, so as to verify which of the four possible cell types was realized.

Crystals of type A show a type-I cell and systematic absences indicate space group P121/a1. Crystals of type B have a type-II cell; in this case, the systematic absences suggest space group P2111 (P21/m11 may be ruled out, following Merlino & Perchiazzi 1988). Finally, crystals of type C display a type-IV cell. No systematic absences were observed, in agreement with a possible P-1 space group. Therefore, the identifica-tion of hiortdahlite II given by Robles et al. (2001) is

incorrect, as hiortdahlite II has a type-I cell, whereas crystals from specimen 119.115 display a type-IV cell.

Qualitative EDS chemical analyses, collected using a Philips XL30 SEM, equipped with an EDAX DX4 system, show the following chemical features: (1) the A type shows Na ≈ Ca, Zr >> Nb; Mn > Ti ≈ Fe; the analysis of a crystal of specimen LOS 180 showed a remarkably greater Ti content than those shown by the other two specimens (106.211 and 131.80); (2) the B type shows Ca > Na; Zr ≈ Nb; minor amounts of Mn, Fe, and Ti; and (3) the C type shows Ca > Na; Zr >> Nb; minor amounts of Mn, Fe, and Ti.

Combining the information resulting from X-ray diffraction studies and qualitative chemical data, it is possible to conclude that crystals of type A correspond to members of the series låvenite-normandite, whereas types B and C correspond to wöhlerite and hiortdahlite I, respectively.

eLeCtrOn-MiCrOPrOBe anaLyses

Quantitative chemical analyses were performed using a JEOL JXA-8600 electron-microprobe, operating in wavelength-dispersion mode; the voltage was 15 kV, the beam current was 20 nA, and the beam diameter was set at 5 μm. The following standards were used: titanite (SiKα, CaKα, TiKα), albite (NaKα), orthoclase (KKα, AlKα), MnTiO3 (MnKα), FeS2 (FeKα), diopside (MgKα), monazite (CeLα, ThMα, PrLβ, SmLβ), La-Re oxide (LaLα), Nd-Cu alloy (NdLβ), UO2 (UMα), REE1 (GdLβ), REE2 (YbLα), REE4 (DyLβ, ErLβ), zircon (ZrLα), yittrium (YLα), metallic niobium (NbLα), metallic tantalum (TaMα), HfO2 (HfMα), topaz (FKα). Corrections were calculated according to the ZAF procedures.

Results of electron-microprobe analyses are reported in Table 2. No quantitative data were collected for the specimen LOS 180, owing to the very small amount of available material.

Låvenite – normandite

According to Perchiazzi et al. (2000), the members of the låvenite-normandite series have a chemical composition intermediate between endmembers NaCaMnTi(Si2O7)OF, corresponding to normandite, and NaNaMZr(Si2O7)F2, i.e., låvenite, where M is a mixed site hosting Mn, with minor Fe, Ca, and Ti.

The same authors report how sodium and calcium cations filling the mixed Na and Ca sites are always in excess with respect to the expected value of 2 atoms per formula unit (apfu). Moving from “normanditic” to “låvenitic” compositions, the sodium amount increases, according to the mechanism Na+ + F- → Ca2+ + O2-, coupled with the substitution Zr4+ → Ti4+ (Fig. 2a). Therefore, a heterovalent substitution Na+ + Zr4++ F- → Ca2+ + Ti4+ + O2- can be discussed (Fig. 2b).


Perchiazzi et al. (2000) reported that, moving from normanditic to låvenitic compositions, there is a remarkable drop in the manganese content together with a constant excess of cations filling the Na, Ca, and (Zr,Ti) sites. Therefore, in agreement with the above considerations, a full-occupancy for the Mn site could be achieved through the double substitution Mn2+ + Ti4+ → Ca2+ + Zr4+ (Fig. 2c), or with the substitution Mn2+

+ Ca2+ + F- → Ti4+ + Na+ + O2- (Fig. 2d). The chemical data obtained from the studied speci-

mens indicate that låvenite from the Los Archipelago is enriched in titanium and manganese, in agreement with the previous study of Moreau et al. (1996). The chem-ical formula, recalculated on the basis of 18 anions, is Na2.04(Ca1.12Na0.84REE0.04)(Mn1.38Ti0.33Fe0.29Ca0.04 Mg0.02)Σ2.07(Zr1.48Nb0.38Ti0.13Hf0.01)(Si1.89O7)2O1.52F2.48.

Wöhlerite

The crystal chemical formula of wöhlerite is matter of debate; according to the structural study

by Mellini & Merlino (1979), it should be written as Na2Ca4ZrNb(Si2O7)2O3F, whereas according to Chakhmouradian et al. (2008), it should be written as Na2Ca4(Zr,Nb)2(Si2O7)2(O,F)4, wöhlerite repre-senting the Zr-analogue of marianoite, Na2Ca4(Nb,Zr)2 (Si2O7)2(O,F)4.

It could be hypothesized that the substitution of Zr4+ by Nb5+ could take place through the coupled substitu-tion Zr4+ + F- ↔ Nb5+ + O2-. However, no clear rela-tion is observed between zirconium and niobium in the chemical data for wöhlerite (Fig. 3a), suggesting that these two elements do not occupy the same structural position.

Wöhlerite from the Los Archipelago shows the highest manganese content ever measured in this mineral, the average MnO content being 5.17 wt.% (Table 2); the measured fluorine content is also high.

Chemical data for wöhlerite (Figs. 3b,c,d,e) indicate a negative correlation between niobium and fluorine, niobium and manganese, and a positive relationship between manganese and fluorine, suggesting the mecha-

TABLE 2. ELECTRON-MICROPROBE ANALYSES OF THE MINERALS OF THE WÖHLERITE GROUP FROM THE LOS ARCHIPELAGO

låvenite wöhlerite hiortdahlite I

wt.%(average n

= 8)

apfu(O + F)

= 18

wt.%(average n

= 10)

apfu(O + F)

= 18

wt.%(average n

= 19)

apfu(O + F) = 18

Nb2O5 6.10(23) 0.386 8.28(62) 0.507 1.08(18) 0.063Ta2O5 n.d. n.d. n.d.SiO2 26.93(29) 3.772 27.27(17) 3.696 28.36(27) 3.688TiO2 4.30(55) 0.453 1.63(21) 0.166 1.17(5) 0.114ZrO2 21.70(101) 1.482 13.78(38) 0.911 14.01(44) 0.888HfO2 0.36(3) 0.014 0.28(5) 0.011 0.21(4) 0.008ThO2 0.03(5) 0.001 0.01(2) 0.000 0.02(3) 0.001Al2O3 n.d. 0.03(5) 0.005 n.d.Y2O3 0.19(6) 0.014 0.41(10) 0.030 0.58(10) 0.040La2O3 n.d. 0.06(5) 0.003 0.08(5) 0.004Ce2O3 0.06(6) 0.003 0.18(8) 0.009 0.18(7) 0.009Pr2O3 0.04(6) 0.002 0.17(11) 0.008 0.13(9) 0.006Nd2O3 0.03(4) 0.002 0.05(5) 0.002 0.07(6) 0.003Sm2O3 0.12(19) 0.006 0.12(19) 0.006 0.12(18) 0.005Gd2O3 0.09(9) 0.004 0.10(8) 0.004 0.04(7) 0.002Dy2O3 0.07(8) 0.003 0.08(8) 0.003 0.07(10) 0.003Er2O3 0.06(6) 0.003 0.02(5) 0.001 0.05(8) 0.002Yb2O3 0.06(6) 0.003 0.11(10) 0.005 0.10(9) 0.004CaO 7.74(37) 1.161 27.27(64) 3.960 36.57(54) 5.095MgO 0.09(1) 0.019 0.59(7) 0.119 0.27(1) 0.052MnO 11.66(74) 1.383 5.17(22) 0.593 2.66(14) 0.293FeO 2.48(59) 0.290 1.09(21) 0.124 1.36(20) 0.148Na2O 10.59(19) 2.876 6.59(29) 1.732 5.21(28) 1.314K2O n.d. n.d. 0.01(2) 0.002F 5.60(35) 2.480 6.48(36) 2.777 10.12(52) 4.162Sum 98.32(96) 99.77(73) 102.46(85)O = F –2.36 –2.73 –4.26Total 95.96 97.04 98.20


nism: Nb5+ + 2Na+ + O2- → Mn2+ + 2Ca2++ F-, in which manganese could enter into the wöhlerite structure by substituting for some niobium.

The chemical formula of the Mn-rich wöhlerite from the Los Archipelago may be written, following Mellini & Merlino (1979), as (Na1.73Ca0.20REE0.07)(Ca3.76Mn0.31Fe0.12)Σ4.19(Zr0.92 Mn0.08)(Nb0.51Mn0.20Ti0.17 Mg0.12)(Si1.85O7)2O1.22F2.78.

Hiortdahlite I

According to Merlino & Perchiazzi (1985), the ideal chemical formula of hiortdahlite I is Na(Na,Ca)Ca4MZr (Si2O7)2OF3, its chemical variability being confined to three sites, indicated as NaCa, M, and F3 in their paper. The substitution Na+ + F- → Ca2+ + O2- describes the variation in occupancy of the mixed NaCa site and of the anion F3 site. Merlino & Perchiazzi (1985), on the basis of the chemical data of Cleve (Brögger 1890), stated that the average charge for the cations at the M site in hiortdahlite I from Langesundsfjord is three, assuming a site occupancy 1/3 Zr4+, 1/6 Ti4+, 1/6 Ca2+, 1/6 Mn2+, 1/6 Fe2+.

The chemical formula for hiortdahlite I from the Los Archipelago, recalculated on the basis of 18 anions, is:

Na1.04(Ca0.66Na0.27REE0.07)Ca4(Ca0.44Mn0.29Fe0.15 Ti0.07Mg0.05)(Zr0.89Nb0.06Ti0.04Hf0.01)(Si3.68O13.84)F4.16. The M site is therefore almost fully occupied by divalent cations (Ca2+, Mn2+, Fe2). Available chemical data for hiortdahlite I point (Fig. 4) to the substitution scheme Zr4+ + Na+ + O2- → 2(Ca2+, Mn2+, Fe2+) ,+ F-; the point deviating from the linear relationship represents a specimen of “hiortdahlite” from Tuva (Kapustin & Bykova 1965), whose precise nature is uncertain.

Taking into account the chemical data from the literature for hiortdahlite I from different localities reported in Merlino & Perchiazzi (1985) together with the data given by Robles et al. (2001), hiortdahlite I from the Los Archipelago is determined to be the poorest in zirconium and sodium and the richest in calcium and fluorine, and shows one of the highest manganese contents ever recorded in this mineral phase (up to 0.34 Mn apfu).

fiG. 2. Chemical relationships (in apfu) in the låvenite-normandite series. The empty squares indicate literature data after Mellini (1981) and reference therein, after Chao & Gault (1997), and Perchiazzi et al. (2000); empty triangles represent the chemical data of låvenite from Los Archipelago given by Moreau et al. (1996). Finally, black triangles show the chemistry of the studied specimens.


CrystaL struCture refineMents

Intensity data collection and crystal structure refinements

The intensity data for the minerals belonging to the wöhlerite group were collected from specimens 106.211, LOS 180, 113.119, and 119.115, corresponding to låvenite, normandite, wöhlerite, and hiortdahlite I, respectively.

Intensity data for låvenite, normandite, and wöhlerite were collected using a Bruker Smart Breeze diffractom-eter, operating at 50 kV and 30 mA, equipped with an air-cooled CCD area detector, at the Dipartimento di Scienze della Terra, Pisa University. Graphite-mono-chromated MoKα radiation was used and the detector-

to-crystal working distance was 50 mm. Frames were collected using φ and ω scan mode, in 0.5° slices. The number of frames and the exposure time for låvenite, normandite, and wöhlerite were respectively 1634 frames and 10 seconds per frame, 1652 frames and 4 seconds per frame, and 1673 frames with 5 seconds exposure per frame. Intensity data were integrated and corrected for Lorentz, polarization, background effects, and absorption using the APEX2 software package (Bruker AXS Inc., 2004).

The intensity data for hiortdahlite I were collected using a Siemens P4 four-circle diffractometer with graphite-monochromated MoKα radiation at the CIADS (Centro Interdipartimentale di Analisi e Determin-azione Strutturale) of Siena University; intensity data were collected in θ-2θ scan mode, scan width 1°, scan

fiG. 3. Chemical relationships (in apfu) in wöhlerite. Empty squares indicate literature data after Sveshnikova & Burova (1973), Mellini & Merlino (1979), Mariano & Roeder (1989), Keller et al. (1995), Chao & Gault (1997), Bellezza (2004), and Chakhmouradian et al. (2008); black triangles indicate the composition of wöhlerite from the Los Archipelago.


speed 2°/min, and corrected for Lorentz-polarization and absorption effect with the ψ-scan method (North et al. 1968).

The crystal structures of låvenite, normandite, wöhlerite, and hiortdahlite I were refined using SHELX-97 (Sheldrick 2008), starting from the atomic coordinates given by Mellini (1981), Perchiazzi et al. (2000), Mellini & Merlino (1979), and Merlino & Perchiazzi (1985), respectively. Scattering curves for neutral atoms were taken from the International Tables for X-ray Crystallography (1992). Crystal data and details of intensity data collection and refinement are reported in Table 3.

For låvenite, the occupancies of the four cation sites were refined using the following curves: Zr vs Ti for the Zr site; Mn vs Ti for the Mn site; and Ca vs Na for the Ca and Na sites. After introducing the anisotropic displacement parameters for the cations, and assuming a normal twinning with twin plane (100), R1 converged to 0.057, thus validating the initial structural model; the ratio between the twinned domains refined to 0.92(1):0.08(1). In the final steps of the refinement, a mixed O,F occupancy was introduced for the O8 site, as suggested by chemical data and by bond-valence balance.

Specimen LOS 180 has a type-I cell; its Ti-rich qual-itative composition suggests that it could be normandite. Occupancies of the four cation sites were refined using the same curves used for låvenite. After ten cycles of isotropic refinement, the structure was completed taking into account the anisotropic displacement parameters. The refinement converged to R1 = 0.022, confirming the validity of the structural model.

During the refinement of the crystal structure of wöhlerite, the occupancies of the eight independent

cation sites were refined using the following curves: Ca vs for the Ca1 and Ca3 sites; Nb vs Ti for the Nb site; Zr vs Mn for the Zr site; Ca vs Fe for the Ca2 and Ca4 sites; and Ca vs Na for the Ca5 and Ca6 sites. After introducing the anisotropic displacement param-eters, R1 converged to 0.019. In the final steps of the refinement, oxygen atoms were partially substituted by fluorine atoms at sites O13, O14, and O16, as suggested by the bond-valence sum.

For the refinement of hiortdahlite I, the occupan-cies of the eight independent cation sites were refined using the following curves: Ca vs for the Ca1, Ca2, and Ca3 sites; Ca vs Na for the Ca4 site; Zr vs Ti for the Zr site; Mn vs Ca for the M site; Na vs for the Na site; and Na vs Ca for the NaCa site. Assuming isotropic displacement parameters and fixed site-occupancies, ten cycles of refinement converged to R1 = 0.20. After refining the site-scattering values and adding (100) normal twinning, the R1 value dropped to 0.075. The ratio between the two twinned domains was 0.53(1):0.47(1). Finally, after introducing anisotropic displacement parameters for all the cations, the final R1 value was 7.3%. In the final steps of the refinement, a partial substitution of oxygen by fluorine at the O7 site was introduced, in agreement with the bond-valence balance and chemical data.

Tables of structure factors, cif files and selected bond-distances (Tables S.1 to S.5; see below) are available at the Depository of Unpublished Data of the Mineralogical Association of Canada website [docu-ments Wöhlerite group CM50_593].

Crystal Structures Details

As previously stated, the crystal structure of the members of the wöhlerite group can be described in modular terms, as composed of disilicate groups and walls of “octahedra”.

Selected bond distances for disilicate groups and “octahedral” cations for each refined structure are reported in Tables S.1 and S.2 (deposited).

In agreement with Merlino & Perchiazzi (1988), disilicate groups are always chelated to those polyhedra hosting large-radius and low-charge cations; Si-O bond-lengths and Si-O-Si bond-angles are in good accord with values previously reported in literature.

Table 4 reports the site-scattering values, the proposed site-populations of the walls of “octahedra”, and the calculated and observed mean bond-lengths. Site-population assignments are based on the electron-microprobe chemical data. Owing to the absence of quantitative chemical analysis for normandite, the corresponding site-scattering values are not reported in the table. However, they clearly agree with the occu-pancy of the Ti site by titanium and minor zirconium, with a refined site-occupancy of Ti0.72(1)Zr0.28(1), corre-sponding to 25.6 electrons per formula unit (epfu), to be compared with 35.2 epfu for låvenite.

fiG. 4. Chemical relationships between (Zr4++Na+) versus (Ca2+ +Mn2++Fe2+) in hiortdahlite I. The empty squares indicate literature data after Merlino & Perchiazzi (1985) and reference therein; the empty triangle represents the chemical data of hiortdahlite from Los Archipelago given by Robles et al. (2001). Finally, black triangles show the chemistry of the studied specimens.


Låvenite and normandite have four independent “octahedral” sites (Figs. 5a and 5b). The Na and Ca sites host sodium and calcium cations. The Mn site has a mixed occupancy by manganese with minor titanium, iron, and probably calcium. Finally, the Zr (in låvenite) and Ti (in normandite) sites have a mixed Zr/Ti occu-pancy, with Zr > Ti in the former and Ti > Zr in the latter. The greater the Ti content, the smaller the average bond-length, changing from 2.07 Å in the Zr-centered octahedron to 2.00 Å in the Ti-centered octahedron. Analogously, the distortion of this polyhedron increases from låvenite to normandite, owing to the off-center displacement (Megaw 1968) related to the occurrence of the small-radius and high-charge Ti4+ cation. According to Subbotin et al. (2000), zirconium and titanium may play a different crystal chemical role owing to their different effective charge, giving rise to a quite limited

isostructurality between Zr- and Ti-analogues. In agree-ment with this observation, for example, no correlation between Zr4+ and Ti4+ was observed in Zr-rich cuspi-dine from Pian di Celle, Umbria, Italy (Bellezza et al. 2004b). On the contrary, taking into account the series låvenite-normandite, a negative correlation between Zr and Ti is clearly discernible (Fig. 2a). One possible explanation to this apparently conflicting behavior comes from the observation of definitely higher manga-nese content in normandite with respect to låvenite. We would also remark that the only other member of the wöhlerite group with titanium as the dominant cation at small octahedral sites is janhaugite, which contains a significant amount of manganese, with three indepen-dent Mn sites in its structure. In this perspective, in the normandite structure the edge of a Ti-centered polyhe-dron would comfortably fit the edge of a Mn-centered

TABLE 3. CRYSTAL DATA AND SUMMARY OF PARAMETERS DESCRIBING DATA COLLECTION AND REFINEMENT FOR LÅVENITE, NORMANDITE, WÖHLERITE, AND

HIORTDAHLITE I

låvenite normandite wöhlerite hiortdahlite I

Crystal dataCrystal size (mm3) 0.20 × 0.08 ×

0.050.45 × 0.06 × 0.04

0.17 × 0.10 × 0.04

0.50 × 0.40 × 0.36

Cell setting, space group

Monoclinic, P21/a

Monoclinic, P21/a

Monoclinic, P2111

Triclinic, P-1

Unit-cell dimensionsa (Å) 10.8475(6) 10.8044(7) 10.80498(16) 10.991(7)b (Å) 9.9364(6) 9.7945(7) 10.25458(13) 10.934(3)c (Å) 7.1488(4) 7.0532(5) 7.28606(10) 7.366(2)α (°) 109.60(3)β (°) 108.392(4) 108.056(3) 109.1168(6) 109.43(2)γ (°) 83.55(3)V (Å3) 731.17(7) 709.64(8) 762.78(2) 786.4(6)Z 2 2 2 2

Data collection and refinementRadiation type, (λ) MoKα (0.71073

Å)MoKα (0.71073 Å)

MoKα (0.71073 Å)

MoKα (0.71073 Å)

Temperature (K) room room room roomMaximum observed 2θ(°)

64.99 65.22 65.17 50.03

Measured reflections 9624 9533 10041 3129Unique reflections 2665 2625 5265 2664Reflections Fo>4ΣFo 2301 2369 5013 2360Rint 0.0303 0.0213 0.0141 0.0444RΣ 0.0316 0.0195 0.0246 0.0442Range of h, k, l -16 ≤ h ≤ 16

-14 ≤ k ≤ 13-10 ≤ l ≤ 10

-16 ≤ h ≤ 15-14 ≤ k ≤ 14-10 ≤ l ≤ 10

-16 ≤ h ≤ 16-15 ≤ k ≤ 15-11 ≤ l ≤ 9

-1 ≤ h ≤ 12-12 ≤ k ≤ 11-8 ≤ l ≤ 8

R1 [Fo > 4Σ Fo] 0.0571 0.0217 0.0192 0.0728R1 (all data) 0.0711 0.0265 0.0212 0.0841wR2 (on Fo

2) 0.1646 0.0536 0.0458 0.1598Goof 1.145 1.089 1.028 1.137Number of l.s. parameters

154 154 280 187

Δρmax and Δρmin 2.94, –1.19 1.07, –0.99 0.83, –1.05 1.07, –1.36


octahedron in an adjacent column, whereas in the låvenite structure, or when a Zr/Ti substitution happens in normandite, a larger Ca-centered polyhedron would instead fit with the edge of a larger Zr octahedron hosted in an adjacent column.

Wöhlerite has eight independent cation sites, owing to their non-centrosymmetric distribution in the walls of “octahedra” (Fig. 5c). Taking into account the refined site-scattering factor and the geometrical features (Table 4), the sites hosting large-radius and small-charge cations, like calcium and sodium, do not show significant differences from those described by Mellini & Merlino (1979) for wöhlerite; in the same way, the Zr site has only a slightly lower electron-density, compat-ible with a minor substitution by a lighter cation. In addition, the Zr-centered octahedron is a nearly regular polyhedron, with five short bonds (average bond-length 2.060 Å) and a longer bond (2.173 Å), quite similar to the Zr polyhedron of wöhlerite described by Mellini & Merlino (1979) and the M2 octahedron of marianoite (Chakhmouradian et al. 2008). On the contrary, the Nb site shows (Table 4) a refined site-scattering definitely lower than the expected value, indicating a significant substitution of niobium by lighter cations. Moreover, the geometry of the Nb-centered octahedron of the Mn-rich wöhlerite from the Los Archipelago is rather

different from the Nb polyhedron in wöhlerite and the M1 polyhedron in marianoite. In fact, whereas the Nb-centered octahedron of wöhlerite from Brevig (Mellini & Merlino 1979) and the M1 polyhedron of marianoite (Chakhmouradian et al. 2008) have average bond distances of 2.033 Å and 2.031 Å respectively, the Nb octahedron of the wöhlerite from Los Archipelago is larger, with an average bond length of 2.073 Å (see Supplementary Table S.2). These data point to the substitution of the small radius Nb5+ cations by lighter and larger ones, such as Mn2+ and Mg2+. Full occupancy of Nb site was achieved by introducing all the titanium at this site, given its similar off-centre displacement effect. The remaining available manganese was added to the Zr site, achieving the full occupancy of this site.

Hiortdahlite I has eight independent cation sites, located in two structurally different “walls” (Fig. 6a), a first “wall” being formed by Ca1, Zr, Ca4, and Na polyhedra (Fig. 6b), and a second one hosting Ca, Ca3, NaCa, and M polyhedra (Fig. 6c). Comparing hiortdahlite I from the Los Archipelago with hiort-dahlite I from Langesundsfjord, Norway, as described by Merlino & Perchiazzi (1985), it is apparent that the most relevant difference is related to the site-occupancy of the M octahedron. Refined site-scattering factors and electron-microprobe analysis point to a site-occupancy

TABLE 4. REFINED SITE-SCATTERING VALUES (epfu) AND ASSIGNED SITE-POPULATION (apfu) FOR CATION SITES IN LÅVENITE, WÖHLERITE, AND

HIORTDAHLITE I

Site Site-scattering Site-population* Calculated

site-scattering <X-φ>calc* Å

<X-φ>obsÅ

låveniteZr 35.2 Zr0.74Nb0.19Ti0.07 38.9 2.07 2.07Mn 24.4 Mn0.67Ti0.16Fe0.14Ca0.02Mg0.01 24.4 2.14 2.22Ca 16.4 Ca0.58Na0.42 16.2 2.50 2.53Na 13.0 Na1.00 11.0 2.52 2.57

wöhleriteCa1 19.8 Ca1.00 20.0 2.49 2.52Nb 30.8 Nb0.51Mn0.20 Ti0.17Mg0.12 31.1 2.05 2.07Zr 38.2 Zr0.92Mn0.08 38.8 2.10 2.08Ca2 22.2 Ca0.64Mn0.26Fe0.10 21.9 2.32 2.31Ca3 19.7 Ca1.00 20.0 2.43 2.43Ca4 20.2 Ca1.00 20.0 2.37 2.37Na1 12.6 Na0.80Ca0.20 12.8 2.51 2.56Na2 11.8 Na0.90Ca0.10 11.9 2.52 2.58

hiortdahlite ICa1 19.8 Ca1.00 20.0 2.49 2.50Ca2 19.8 Ca1.00 20.0 2.37 2.36Ca3 21.7 Ca1.00 20.0 2.37 2.34Ca4 18.4 Ca1.00 20.0 2.42 2.43Zr 36.0 Zr0.90Nb0.05Ti0.05 39.2 2.08 2.11M 23.4 Ca0.44Mn0.29Fe0.15Ti0.07Mg0.05 22.1 2.23 2.26Na 13.5 Na0.95REE0.05 13.0[1] 2.48[2] 2.51NaCa 15.5 Ca0.65Na0.35 16.8 2.49 2.54

* Ionic radii from Shannon & Prewitt, 1969; [1] mean scattering curve of 49.94 for Y3+ and REE3+; [2] ionic radius of [VII]Na+


by calcium, manganese, iron, with minor titanium and magnesium, with an average charge of 2.14, compared with the average charge of 3 for hiortdahlite I from Norway (Merlino & Perchiazzi, 1985); the lower charge of the M site is in agreement with the high fluorine content of hiortdahlite I from the Los Archipelago.

The site-occupancy of anions in the refined crystal structures (reported in Tables S.3, S.4, and S.5, depos-ited) was inferred on the basis on bond-valence sums, in agreement with Brese & O’Keeffe (1991).

In the crystal structures of låvenite and normandite nine independent anion sites are present. The bond-

valence balance (Table S.3) suggest that O1-O7 sites are occupied by oxygen, whereas F9 is a fluorine-bearing site. O8 is a mixed O,F site in låvenite, in agreement with Mellini (1981), whereas it is fully occupied by oxygen in normandite.

Eighteen independent anion sites occur in the crystal structures of wöhlerite and hiortdahlite I. In wöhlerite from the Los Archipelago (Table S.4), O1 to O12 and O17 and O18 are pure oxygen sites, and the full occu-pancy of F15 by fluorine is confirmed. The O13, O14, and O16 sites are undersatured, suggesting a mixed O,F occupancy, in agreement with electron-microprobe data.

fiG. 5. The “octahedral” walls of låvenite (a), normandite (b), and wöhlerite (c). Låvenite and normandite are seen down [100]; b is horizontal and c is vertical. Wöhlerite is seen down [010]; c is vertical.


It is noteworthy that these three latter sites are bonded to the Nb-centered octahedron, in which manganese partially substitutes for niobium, through the scheme Nb5+ + 2Na+ + O2- → Mn2+ + 2Ca2+ + F- (Fig. 3d). In the refined hiortdahlite I structure from the Los Archi-pelago, bond-valence calculation (Table S.5) indicate a full oxygen occupancy of O1 to O6 and O8 to O15 sites, a full fluorine occupancy at F1, F2, and F3 sites, and a mixed occupancy by oxygen and fluorine at the O7 site.

Partitioning of zirconium and niobium in wöhlerite

The results obtained during the refinement of the crystal structure of Mn-rich wöhlerite from the Los Archipelago allow us to draw some conclusions about the different crystal chemical behavior of niobium and zirconium in wöhlerite.

As previously reported, Mellini & Merlino (1979) and Chakhmouradian et al. (2008) proposed two different crystal-chemical formulae for wöhlerite, Na2Ca4ZrNb (Si2O7)2O3F or Na2Ca4(Zr,Nb)2(Si2O7)2O3F, respec-tively. The difference is related to the possible ordering of (Nb+Ti) and Zr at the two smallest octahedrally coor-dinated sites. Owing to the very similar X-ray scattering factors, the ratio of zirconium and niobium at these sites cannot be refined directly using radiation routinely employed in laboratory equipment, such as MoKα. According to Mellini & Merlino (1979), the distinct occupancy of these two sites is suggested by: (1) their different size, (2) their different distortion, and (3) their different equivalent thermal parameters. Chakhmoura-dian et al. (2008) criticized these arguments, stating that there is no convincing empirical evidence for the ordering of niobium and zirconium at two distinct sites

fiG. 6. The crystal structure of hiortdahlite I. (a) Crystal structure seen down [001]; (b) and (c) show the two kinds of “octahedral” walls.


and that the extensive substitutions in different members of the wöhlerite group attest to a great structural flex-ibility of these phases. Therefore, according to these latter authors, wöhlerite would contain zirconium domi-nant at both sites, whereas marianoite would contain niobium dominant at both sites.

The structural data for Mn-rich wöhlerite presented in this study can be examined to obtain evidence supporting one of these two contrasting interpreta-tions. As the geometry of a coordination polyhedron is controlled by several parameters, both intrinsic (e.g., the site occupancy) and extrinsic (i.e., the electrostatic forces exerted by ions outside the nearest coordination sphere), it is particularly interesting to compare poly-hedra in the crystal structure of the same mineral group.

The size and the degree of distortion of the small octahedra, preferentially occupied by zirconium, niobium, and titanium in the minerals of the wöhlerite group, can be evaluated through the comparison of some parameters, reported in Table 5 together with the site occupancies of the above-mentioned small octahedra. Merlino & Mellini (2009) used the parameter Δ as an indication of the degree of distortion. According to Robinson et al. (1971), the measure of the distor-tion of coordination polyhedra can be also expressed through two parameters, namely the bond angles vari-ance (Σ2) and the mean quadratic elongation (λ). For a coordination octahedron, Σ2 = (1/11) Σ (θi-90°)2 and

λ = (1/6) Σ (li/l0)2, where θi are the bond angles, li are the bond-lengths, and l0 is the bond-length for the ideal undistorted octahedron equal in volume to the one in question. Figures 7, 8a, and 8b illustrate the relation between the zirconium content and some geometrical parameters, such as the average bond-length, the polyhedron volume, the octahedral angle variance Σ2, the mean octahedral quadratic elongation λ, and the Δ parameter. One can notice that the Zr-dominant sites clearly form a distinct group, with longer average bond-lengths, greater polyhedron volumes, and lower Δ, Σ2, and λ values. The only exception is represented by the Zr-centered octahedron of baghdadite, which has higher values of both Σ2 and λ. These geometric features, shown also by the synthetic counterpart studied by Plaisier et al. (1995), are probably induced by the unusual cation distribution in its walls of “octa-hedra”, with two Zr polyhedra connected through edge-sharing. The (Nb,Ti)-dominant sites constitute, on the other hand, a group with significantly shorter bond-lengths, smaller polyhedron volumes, and greater Σ2, λ, and Δ values. The Nb site in wöhlerite from the Los Archipelago represents an exception, owing to its high manganese content. Marianoite was not reported in these plots, because Chakhmouradian et al. (2008) did not report the site occupancies of the two small octahedral sites M1 and M2. In Figure 8c the relations between two geometrical features, namely Σ2 and λ, are

TABLE 5. GEOMETRICAL PARAMETERS OF NB, TI, AND ZR SITES IN MINERALS BELONGING TO THE WÖHLERITE GROUP

Site Site occupancyaverage bond-length (Å)

average edge-length (Å)

average angle (°)

polyhedral volume (Å3) Σ2 λ Δ Ref.

baghdadite Zr Zr0.86Ti0.14 2.104 2.961 105.84 11.962 82.61 1.026 0.23 [1]burpalite Zr Zr 2.097 2.962 107.06 12.196 17.89 1.005 0.04 [2]hiortdahlite I Zr Zr 2.089 2.951 106.94 12.031 23.00 1.008 0.18 [3]hiortdahlite I Zr Zr0.90 Nb0.05Ti0.05 2.110 2.979 106.74 12.352 29.03 1.010 0.19 [4]hiortdahlite II Zr Zr 2.149 3.035 106.89 13.080 25.28 1.008 0.09 [5]

janhaugite Ti1 Ti0.7Zr0.3 2.015 2.842 105.95 10.736 37.15 1.014 0.37 [6]Ti2 Ti0.7Zr0.3 2.010 2.832 105.53 10.608 45.68 1.019 0.44låvenite Zr Zr0.86Nb0.14 2.083 2.941 106.49 11.910 25.22 1.009 0.21 [7]låvenite Zr Zr0.74Nb0.17Ti0.09 2.069 2.922 106.53 11.691 24.06 1.009 0.25 [4]

marianoite M1 2.031 2.858 105.64 10.962 44.07 1.019 0.44 [8]M2 2.080 2.938 106.86 11.884 22.13 1.007 0.10niocalite Nb Nb 2.049 2.884 105.67 11.180 60.08 1.022 0.37 [9]normandite AF Ti Ti0.61Zr0.39 2.028 2.860 106.06 10.962 34.02 1.014 0.41 [10]normandite MSH Ti Ti0.88Nb0.12 2.003 2.821 105.80 10.534 39.50 1.018 0.52 [10]

normandite Ti Ti0.75Zr0.25 2.000 2.818 105.81 10.497 38.55 1.017 0.51 [4]

wöhlerite Nb Nb0.8Ti0.2 2.034 2.865 105.83 11.031 39.65 1.017 0.42 [11]Zr Zr 2.083 2.944 107.02 11.965 17.43 1.005 0.12

wöhlerite Nb Nb0.51Mn0.20 Ti0.17Mg0.12

2.073 2.922 106.47 11.667 36.77 1.014 0.34[4]

Zr Zr0.92Mn0.08 2.078 2.934 106.97 11.848 18.36 1.006 0.14

[1] Biagioni et al. (2010b); [2] Merlino et al. (1990); [3] Merlino & Perchiazzi (1985); [4] this work; [5] Merlino & Perchiazzi (1987); [6] Annehed et al. 1985; [7] Mellini (1981); [8] Chakhmouradian et al. (2008); [9] Mellini (1982); [10] Perchiazzi et al. (2000); [11] Mellini & Merlino (1979).


shown: two distinct groups can be identified, i.e., one group including the Zr-dominant sites, and the other comprising the (Nb,Ti)-dominant sites. It is interesting to note that the M2 and the M1 sites of marianoite fall within the former and the latter groups, respectively. Figure 8c therefore suggests that, in the wöhlerite group, (Nb,Ti) and Zr sites can be distinguished on the basis

of geometrical parameters. In the light of these data, marianoite could be considered a wöhlerite with M1 and M2 sites dominated by niobium and zirconium, respectively. As suggested by Merlino & Mellini (2009), studies through anomalous scattering, with synchrotron X-ray radiation of appropriate wavelength, would allow direct investigation of the Zr/Nb distribution in the octahedral sites of wöhlerite and marianoite.

fiG. 7. Relationships between the site occupancy of the small octahedral sites and the average bond-length (a), polyhedral volume (b), and Δ parameter (c) in minerals of the wöhlerite group. Circles: Nb sites; square: Ti sites; triangles: Zr sites.

fiG. 8. Relationships between the site occupancy of the small octahedral sites and their octahedral angle variance Σ2 (a) and mean octahedral quadratic elongation λ (b). In (c), these two geometric parameters are compared. Circles: Nb sites; square: Ti sites; triangles: Zr sites. In (c), crosses indicate the two small octahedral sites of marianoite.


COnCLusiOns

Coupling X-ray diffraction and chemical data, four different phases belonging to the wöhlerite group were identified in the nepheline syenites from the Los Archipelago: låvenite, normandite, wöhlerite, and hiortdahlite I. In the light of our data, the occurrence of hiortdahlite II, described by Robles et al. (2001) appears quite doubtful.

A chemical feature common to all the investi-gated minerals of the wöhlerite group is their quite high manganese content, this being the highest ever measured in wöhlerite. The wide range of Zr/Ti ratio in the låvenite-normandite series, not detected in other mineral series, could be explained through the coupled substitution of manganese by calcium. Normandite could host contemporaneously Ti- and Mn-centered octahedra in adjacent columns, bonded through edge-sharing, both smaller in size with respect to Zr- and Ca-octahedra occurring in the crystal structure of låvenite. The comparison of the geometry of the (Nb,Ti)- and Zr-centered polyhedra, including all avail-able structural studies of the wöhlerite group, indicates that a distinction is possible between (Nb,Ti)- and Zr-dominant sites in this mineral family.

The results from the present study and the recent discovery of the new mineral roumaite (Biagioni et al., 2010a) in the nepheline syenites from the Los Archi-pelago suggests that, taking into account the richness in mineral species of other famous alkaline complexes, i.e., more than 400 different mineral species reported in the Mont Saint-Hilaire, Khibiny, and Lovozero massifs, further sampling and lab studies will possibly greatly increase the number of mineral species described from this locality.

aCknOwLedGMents

For this research, we received support from the SYNTHESYS Project (FR-TAF-2137: “Crystallog-raphy and crystal chemistry of rare Zr-Ti-Nb phases from Iles de Los, Guinea”; http://www.synthesys.info/), which is financed by the European Community Research Infrastructure Action under the FP6 “Structure the European Research Area” and by MIUR through project PRIN 2007 “Compositional and structural complexity in minerals (crystal chemistry, microstruc-tures, modularity, modulations): analysis and applica-tions”. We are grateful to Editor Robert F. Martin, to Associated Editor Ian Coulson, to Fernando Cámara and to an anonymous referee for the comments and suggestions that helped us in improving the manuscript.

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Received December 22, 2011, revised manuscript accepted June 13, 2012.

CRYSTAL CHEMISTRY OF MINERALS OF THE WÖHLERITE GROUP FROM THE LOS ARCHIPELAGO, GUINEA

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